The ability to view the interior of a human body is often important for understanding and diagnosis of an illness or disease. Preferably, such viewing is done non-invasively to minimize the trauma that may be caused by an invasive method. For example, it is advantageous to view a patient's body without making incisions on the patient or breaking the patient's skin. Ultrasonic imaging equipment, including ultrasonic probes, have found widespread use for diagnosis in medicine.
Although ultrasonic imaging equipment (or system) provides a convenient and accurate way of gathering information from within a body, the accuracy and sensitivity of such ultrasonic systems are affected by the efficient transmission of acoustic energy from the system to a target in the body and the efficient reception of reflected ultrasonic energy by the receiver (such as a transducer). In ultrasonic medical imaging, typically the propagating medium (i.e., tissue) has an acoustic impedance that is different from that of the piezoelectric medium gathering or detecting acoustic signals. Such differences are often large and necessitate a means for acoustic impedance matching. The acoustic signal transmitted through the medium is only weakly reflected by the target (such as the heart in the body). For this reason, efficient acoustic coupling between the ultrasonic probe (including a transducer) and the propagating medium (tissue) is required.
Because the human body is not acoustically homogenous, depending on the target, the location thereof, and the medium, different frequencies of operation of the ultrasonic imaging system may be preferred. For example, an acoustic signal of higher frequency may provide a sharper image, but may not penetrate as deeply in the body as an acoustic signal of lower frequency. Therefore, it is desirable to provide an ultrasonic imaging system having the capability to operate under two different frequencies. U.S. Pat. No. 5,163,436 (Saitoh et al.) disclosed an ultrasonic probe system having a plurality of piezoelectric layers and a polarization control circuit means. The polarization control circuit means controls the polarity of the electric field applied to every two adjacent layers of the piezoelectric layers in the ultrasonic probe system. This is done to control the oscillation resonance frequency of the transducer or its impulse response. The system can be used to select and generate ultrasonic waves having a plurality of different frequencies. However, such a system may still have only limited usable bandwidth at each of it's resonance frequencies and have relatively long ringdown time (which is related to the length of time required for the acoustic energy to dissipate when the oscillation signal has been turned off). A long ringdown time results in unwanted signals interfering with signals of desired information, thus adversely affect the imaging resolution of the imaging system.
A high efficiency ultrasonic transducer with a wide frequency bandwidth enables the production of high quality images. Acoustic impedance-matching between the piezoelectric layer and the propagating medium facilities wide-band operation and high sensitivity. The acoustic impedance mismatch between a piezoelectric layer that generates or receives acoustic energy (such as a piezoelectric ceramic, e.g., lead zirconate titanate (PZT)) and the propagating medium typically requires an impedance-matching layer for better transfer of acoustic energy from the ceramic to the medium. Some traditional ultrasonic probe systems (such as those disclosed in U.S. Pat. No. 5,163,436) have impedance-matching layers. However, the bandwidth of such traditional ultrasonic probe systems are still relatively narrow and the ringdown time is relatively long.